The air bubble in the fuel line does not mean anything and will not effect the operation of the carb as it is a gravity flow system and fuel will just flow past the bubble.
Have you checked the carb float bowl vent? It needs to be clear and open for fuel to flow to the float jet.
Have you cleaned the muffler yet? Have you removed the head and checked the exhaust port? They could both be plugged up if the previous owners were using dino oil.